This invention relates generally to non-destructive examinations of gas turbine engine components, and more particularly to non-destructive evaluations of thermal barrier coatings applied to gas turbine components.
A gas turbine engine includes a compressor for compressing air which is channeled to a combustor and mixed with a fuel, wherein the mixture is ignited for generating hot combustion gases. The combustion gases are channeled to a turbine, which extracts energy from the combustion gases for powering the compressor, as well as producing useful work to propel an aircraft in flight or to power a load, such as an electrical generator. Increased efficiency in gas turbine engines is facilitated at least in part by an increase in the operating temperature of the combustor. A limitation on the operating combustor temperature may be a temperature limitation of combustor liner material.
A thermal barrier coating can be applied to inner surfaces of the combustor liner or other components of the turbine, for example, blades and vanes, for providing thermal insulation against combustion gases. Thermal barrier coatings facilitate reducing an amount of cooling air required for a given combustion gas temperature, or allow an increase in a combustion gas temperature for increasing efficiency of the engine. See, for example, U.S. Pat. No. 5,960,632. Typically the thermal barrier coating is applied uniformly across the turbine component with a thickness of 0.01 inches or less.
Thermal barrier coating systems can include an inner bond coat applied to the metal substrate, an outer thermal insulating layer that includes one or more ceramic materials, and a thin intermediate alumina layer located between the bond coat and the thermally insulating layer to promote adhesion of the thermally insulating layer. The alumina layer is formed during processing and is commonly referred to as a thermally grown oxide. Known thermal barrier coatings include a zirconia stabilized with yttria thermal insulating top layer and a metallic overlay and/or diffusion aluminide bond coat.
Sometimes spalling of the outer thermal insulating layer can occur at the intermediate layer/insulating layer interface or at the intermediate/bond coat interface as a result of coating defects and/or a build-up of stresses at these interfaces. The presence of residual compressive stress in thermal barrier coatings are closely linked to thermal barrier coating adhesion through testing and engine service. Further, it has been shown that compositional and/or microstructual variations in a thermal barrier coating can lead to significant changes/differences in the thermal conductivity of the thermal barrier coating.
Standard non-destructive testing techniques, for example, through transmission ultrasound and X-ray diffraction may be unable to assess thermal barrier coating integrity due to density differences between the substrate and the coating. Known residual stress measurement techniques, such as X-ray diffraction, have limited use in determining thermal barrier coating quality because of the difficulty in penetrating through the thermal insulating layer to the intermediate layer. The intermediate thermally grown oxide layer is also very thin, typically about 1 micron, and therefore is very difficult to characterize by X-ray diffraction and other conventional techniques, for example, neutron diffusion. Another non-destructive measurement method which can measure residual stress proximate an intermediate thermally grown oxide layer in a multi-layer thermal barrier coating directs a laser beam through the outer ceramic thermal insulating layer such that the laser beam illuminates the intermediate layer in a manner to cause species present in the intermediate layer to fluoresce, measuring the frequency of the light or photons emitted by the fluorescing species, and comparing the measured frequency of the intermediate layer to the frequency shift determined on like material under controlled stress states to determine a representation of relative residual stress in the coating. See for example, U.S. Pat. No. 6,072,568. However, these methods are limited and only indirectly measure some properties of the thermal barrier coating and do not provide detailed morphological information needed to asses thermal barrier coating quality.